The essential role of clathrin-mediated endocytosis and early endosomes in the trafficking of begomoviruses through the primary salivary glands of their whitefly vectors.

IMPORTANCE: Many plant viruses are transmitted by insect vectors in a circulative manner. For efficient transmission, the entry of the virus from vector hemolymph into the primary salivary gland (PSG) is a step of paramount importance. Yet, vector components mediating virus entry into PSG remain barely characterized. Here, we demonstrate the role of clathrin-mediated endocytosis and early endosomes in begomovirus entry into whitefly PSG. Our findings unravel the key components involved in begomovirus transport within the whitefly body and transmission by their whitefly vectors and provide novel clues for blocking begomovirus transmission.

Begomovirus-Associated Betasatellite Virulence Factor ßC1 Attenuates Tobacco Defense to Whiteflies via Interacting With Plant SKP1.

Plant-mediated interactions between plant viruses and their vectors are important determinants of the population dynamics of both types of organisms in the field. The whitefly Bemisia tabaci can establish mutualism with begomoviruses via their shared host plants. This mutualism is achieved by the interaction between virulence factors and their host proteins. While the virulence factor ßC1 encoded by tomato yellow leaf curl China betasatellite (TYLCCNB), a subviral agent associated to the begomovirus tomato yellow leaf curl China virus (TYLCCNV), may interact with plant protein MYC2, thereby establishing the indirect mutualism between TYLCCNV and whitefly, whether other mechanisms are involved remains unknown. Here, we found the in vitro and in vivo interactions between ßC1 and tobacco protein S-phase kinase associated protein 1 (NtSKP1). Silencing the expression of NtSKP1 enhanced the survival rate and fecundity of whiteflies on tobacco plants. NtSKP1 could activate the transcription of genes in jasmonic acid (JA) pathways by impairing the stabilization of JAZ1 protein. Moreover, ßC1-NtSKP1 interaction could interfere JAZ1 degradation and attenuate the plant JA defense responses. These results revealed a novel mechanism underlying the better performance of whiteflies on TYLCCNV/TYLCCNB-infected plants.

WRKY Transcription Factors in Nicotiana tabacum Modulate Plant Immunity against Whitefly via Interacting with MAPK Cascade Pathways.

WRKY transcription factors are key regulators of many plant processes, most notably coping with biotic and abiotic stresses. Recently, the function of WRKY in plant defense against phloem-feeding insects such as whitefly (Bemisia tabaci) has been brought to attention. In this study, we found that the expression levels of Nicotiana tabacum WRKY4, WRKY6 and WRKY10 were significantly upregulated when tobacco plants were infested with whiteflies or treated with salicylic acid. Compared to controls, whiteflies lived longer and laid more eggs on NtWRKY-silenced tobacco plants but performed less well on NtWRKY-overexpressing plants. The three NtWRKYs interacted with five mitogen-activated protein kinases (NtMAPKs) in vivo and in vitro. These results suggest that the WRKYs in tobacco positively modulate plant defense against whiteflies through interaction with the mitogen-activated protein kinase cascade (MAPK cascade) pathways, and thus provide new insights into plant defense against phloem-feeding insects.

Transovarial transmission of tomato yellow leaf curl virus by seven species of the Bemisia tabaci complex indigenous to China: Not all whiteflies are the same.

Begomoviruses contain some of the most damaging viral disease agents of crops worldwide, and are transmitted by whiteflies of the Bemisia tabaci species complex. During the last 20 years, transovarial transmission of tomato yellow leaf curl virus (TYLCV) has been reported in two invasive species of the B. tabaci complex. To further decipher the importance of this mode of transmission, we analyzed transovarial transmission of TYLCV by seven whitefly species indigenous to China. TYLCV virions were detected in eggs of all species except one, and in nymphs of two species, but in none of the ensuing adults of all seven species. Our results suggest that these indigenous whiteflies are unable to transmit TYLCV, a begomovirus alien to China, via ova to produce future generations of viruliferous adults, although most of the species exhibit varying ability to carry over the virus to the eggs/nymphs of their offspring via transovarial transmission.

Vector and nonvector insect feeding reduces subsequent plant susceptibility to virus transmission.

The interactions of vector-virus-plant have important ecological and evolutionary implications. While the tripartite interactions have received some attention, little is known about whether vector infestation affects subsequent viral transmission and infection. Working with the whitefly Bemisia tabaci, begomovirus and tobacco/tomato, we demonstrate that pre-infestation of plants by the whitefly vector reduced subsequent plant susceptibility to viral transmission. Pre-infestation by the cotton bollworm, a nonvector of the virus, likewise repressed subsequent viral transmission. The two types of insects, with piercing and chewing mouthparts, respectively, activated different plant signaling pathways in the interactions. Whitefly pre-infestation activated the salicylic acid (SA) signaling pathway, leading to deposition of callose that inhibited begomovirus replication/movement. Although cotton bollworm infestation elicited the jasmonic acid (JA) defense pathway and was beneficial to virus replication, the pre-infested plants repelled whiteflies from feeding and so decreased virus transmission. Experiments using a pharmaceutical approach with plant hormones or a genetic approach using hormone transgenic or mutant plants further showed that SA played a negative but JA played a positive role in begomovirus infection. These novel findings indicate that both vector and nonvector insect feeding of a plant may have substantial negative consequences for ensuing viral transmission and infection.